The subject matter disclosed herein relates to electromagnetic devices and machines and, more particularly, to an arrangement for determining the position of a generator or motor rotor.
In a power conversion system, such as a variable-speed, constant-frequency (VSCF) power generating system, a generator, typically a brushless, three-phase synchronous generator, is operated in a generating mode to convert variable-speed motive power supplied by a prime mover into variable-frequency alternating current (AC) power. The prime mover may be, for example, a gas turbine engine of an aircraft. In some cases, the variable-frequency AC power produced by the generator is transmitted to a rectifier where it is rectified and provided as a direct current (DC) signal over a DC link to an inverter. The inverter may then invert the DC signal on the DC link into constant-frequency AC power for supply over a load bus to one or more AC loads.
A generator can also be operated as a motor in a starting mode to convert electrical power supplied by an external AC power source into motive power which may in turn be provided to the prime mover to bring the prime mover up to self-sustaining speed. For example, when operated in a starting mode, the generator can be used to start a gas turbine engine of an aircraft.
One type of a brushless, synchronous generator, which can be operated in both a generating mode and a starting mode, includes a permanent magnet generator (PMG). In order to properly control such a generator, it is necessary to time the rectification in synchronicity with the position of the generator's rotor. Previously, such synchronization was achieved by using a position sensor such as a Hall Effect sensor. As the rotor of the PMG rotates, the voltage across each Hall Effect sensor varies from zero to a maximum as a function of rotor position such that the outputs from the Hall Effect sensors are representative of the position of the rotor. The output signals from the Hall Effect sensors are then used to control switching elements in the rectifier.
The use of such position sensors, however, entails considerable expense in the position sensor itself and its associated electronics, and further results in extra wires and extra assembly steps to install the position sensing apparatus. Also, operational parameters often limit the accuracy of such position sensors.